Accumulation of sediments, trace metals (Pb, Cu) and total hydrocarbons in Narragansett Bay, Rhode Island

Abstract

The accumulation of sediments, trace metals and hydrocarbons has been estimated from the analysis of the sediment from six coring sites in Narragansett Bay. Radionuclides ( 234Th xs, 210Pb xs, 239,240Pu) with known input functions and trace metals (Cu, Pb) were used. We estimate that 6·9 × 10 4 tons of sediments, 51–90 tons of Pb, 72–100 tons of Cu and 400–1000 tons of total hydrocarbons accumulate annually under present conditions in the bay. This represents 64–117% (Pb), 89–123% (Cu) and 23–58% (hydrocarbons), respectively, of present day inputs to the bay. Furthermore, close to 100% of the particle-reactive radionuclides 210Pb and 239,240Pu accumulate in the bay. Present day inputs to the bay were calculated independently as 77–80 tons Pb and 81 tons of Cu. Sewage effluents were the dominant source of Cu, whereas atmospheric deposition and urban runoff were most important for Pb. Dredging activities by the U.S. Army Corps of Engineers between 1946 and 1971 removed more sediments from the bay than would have accumulated during the same time in the undredged areas of the bay. Copper smelting and coal mining on the shores of the upper bay during 1866–1880 left an imprint in the sediments which is still evident. Model derived accumulation rates of Pb, Cu and coal during that time were 3–4 times present-day inputs.